1. Field of the Invention
The invention relates, in general, to semiconductor fabrication using metalorganic chemical vapor deposition crystal growth, and more particularly to vapor phase transport of a dopant species by organic free radicals, preferably aliphatic radicals such as methyl, ethyl, or n-propyl, for controlled doping in a reproducible manner.
2. Prior Art
The Manasevit U.S. Pat. Nos. 4,368,098 and 4,404,265 describe methods and apparatus for growing thin film single crystal Group III-V wide band-gap compound semiconductors, wherein the thin film may be produced in situ on a heated substrate in an MOCVD reactor by reaction of an organic compound containing a Group III constituent with a Group V hydride. These patents are assigned to a common assignee of the present application, and techniques disclosed in the patents are used in the present invention.
In the manufacturing of electrically active single crystal semiconductor materials by metalorganic chemical vapor deposition (MOCVD), precise control over the carrier concentration in the materials is essential. A common way to control carrier type and concentration is by intentionally doping the material during growth with a dopant species which will provide the correct carrier type at the desired concentration in the crystalline material.
The problem which has existed with present doping schemes in MOCVD growth is a lack of precise control and reproducibility at low dopant concentrations. One of these schemes which exhibits problematic behavior is alkyl-doping. This is where the dopant species is introduced in the form of a presynthesized metalorganic chemical, and is subsequently decomposed in the growth zone of the crystal growth reaction chamber by heat energy which emanates from a localized heat source, as in the Manasevit patents (supra). The dopant flux in this arrangement is controlled by the partial pressure of the metalorganic compound in the gas phase. The partial pressure is established by the concentration of metalorganic compound in the vapor transporting the compound as a diluent gas, hydrogen, through a network of stainless steel tubing which interconnects the chemical source and the reaction chamber. One problem that occurs with this configuration is the intolerable decay of dopant levels due to condensation, and/or adsorption/desorption processes, of the metalorganic chemicals in the transport lines. This is commonly referred to as the "memory effect." It is this memory effect which complicates the issue of fabricating abrupt transitions in electrically active dopant species. This memory effect phenomena may require numerous growth runs, in which the dopant species is not introduced, for the dopant concentration to fall below deleterious levels.